Hearing assistive device with increased dynamic input range

ABSTRACT

A hearing assistive device including an audio processing circuit with an analog-to-digital converter having an integrator integrating a voltage present in the summation point; a comparator comparing an output from the integrator with a reference voltage (V ref ) and outputting a logical level in accordance with the comparison; a feedback loop coupling a feedback signal back to the summation point; and a reference voltage generation circuit being adapted to provide the reference voltage (V ref ) being lower than a power supply voltage (V battery ) and following the decay of the power supply voltage (V battery ) with a predefined margin.

RELATED APPLICATIONS

This application is a bypass continuation of international applicationNo. PCT/EP2016/080925, filed on Dec. 14, 2016, in Europe and publishedas WO2018108260 A1, the disclosures of which are incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to hearing assistive devices. Theinvention, more particularly, relates to a hearing assistive device withincreased dynamic input range. Also, the invention relates to a voltagesupply for an analog-to-digital converter.

When designing a hearing assistive device or a hearing aid, focus is onto increasing the dynamic input range. The dynamic input range is aprerequisite for ensuring a true reproduction of natural sounds. Thepresence of sounds exceeding the dynamic range of the input stage causesthe signal to be clipped, which results in distorting harmonics presentin the signal to be processed.

There is a need for providing excellent sound quality to users ofhearing assistive devices or hearing aids even in very loud soundenvironments, e.g. loud music, parties, walking in busy city streets,etc.

SUMMARY OF THE INVENTION

The purpose of the invention is to provide a hearing assistive device ora hearing aid with increased dynamic input range ensuring thereproduction of natural sounds.

This purpose is according to the invention achieved by a hearingassistive device including an audio processing circuit. The audioprocessing circuit comprises an analog-to-digital converter having anintegrator integrating a voltage present in a summation point; acomparator comparing a voltage output from the integrator with areference voltage and outputting a logical level in accordance with thecomparison; a feedback loop coupling a feedback signal back to thesummation point; and a reference voltage generation circuit beingadapted to provide the reference voltage at a level lower than that of apower supply voltage and following the decay of the power supply voltagewith a predefined margin.

By letting the reference voltage depend of battery supply voltage ratherthan being fixed, the achievable dynamic range from a hearing can beincreased. The increased dynamic range provides artifact-free soundprocessing even in even louder sound environments, and thereby improvedspeech intelligibility in loud sound environments.

According to a second aspect of the invention there is provided areference voltage generation circuit for providing a reference voltagefrom a power supply, and comprising an electronic voltage amplifier(op-amp) coupled to the power supply via a passive circuit, and beingadapted to control the reference voltage to be lower than the powersupply voltage and follows the decay of the power supply voltage with apredefined margin.

According to a third aspect of the invention there is provided an audioprocessing circuit comprises an analog-to-digital converter having anintegrator integrating a voltage present in a summation point, acomparator comparing an output from the integrator with a referencevoltage and outputting a logical level in accordance with thecomparison, a feedback loop coupling a feedback signal back to thesummation point. The audio processing circuit further comprises areference voltage generation circuit being adapted to provide thereference voltage being lower than a power supply voltage and followingthe decay of the power supply voltage with a predefined margin.

According to a fourth aspect of the invention there is provided a methodfor processing audio in a hearing assistive device, and comprising stepsof converting an analog signal from a microphone in an analog-to-digitalconverter, generating a reference voltage from a power supply voltage,the reference voltage being lower than the power supply voltage, andfollowing the decay of the power supply voltage with a predefinedmargin; and supplying the reference voltage to the analog-to-digitalconverter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail with reference topreferred aspects and the accompanying drawing, in which:

FIG. 1 illustrates schematically the operation of a delta sigmaconverter;

FIG. 2 illustrates schematically a hearing assistive device according toan embodiment of the invention;

FIG. 3 illustrates an embodiment for a reference voltage generationcircuit according to the invention;

FIG. 4 illustrates an embodiment of an input transformer utilizedaccording to certain aspects of the invention; and

FIG. 5 illustrates the reference voltage relative to the battery voltageaccording to one aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1, which schematically illustrates a deltasigma converter 10 in which a reference voltage generation circuitaccording to one embodiment of the invention may implemented. The deltasigma converter 10 converts an analog voltage or analog signal 13received on an input 11 into a digital representation 14 delivered on anoutput 12. The digital representation 14 is known as Pulse Densitymodulation or Pulse Frequency modulation. In general, frequency may varysmoothly in infinitesimal steps, as may voltage, and both may serve asan analog of an infinitesimally varying physical variable such as aspeech signal or an acoustic signal. The substitution of frequency forvoltage is thus entirely natural and carries in its train thetransmission advantages of a pulse stream.

Most A/D converters, including the delta sigma converter 10, requires asinput a reference voltage. The input voltage to the A/D converter ismeasured relative to this reference voltage. Hence, it is important thatthe reference voltage has sufficiently low noise.

The delta sigma converter 10 converts the mean of the analog voltageinto the mean of the analog pulse frequency and counts the pulses in aknown interval so that the pulse count divided by the interval gives anaccurate digital representation of the mean analog voltage during theinterval. This interval can be chosen to give any desired resolution oraccuracy.

FIG. 2 illustrates schematically a hearing assistive device according toan embodiment of the invention. The hearing assistive device includes anexample of an A/D converter. This converter is a 1-bit time-continuousdelta sigma converter of first order, but the principles according tothe invention applies to all converter types. The hearing assistivedevice may be a hearing aid.

The hearing assistive device has at least one input transducer ormicrophone 18 picking up an audio signal and transforming it into anelectric representation, e.g. the analog signal 13. The delta sigmaconverter 10 receives the analog signal 13 at the input 11. In oneembodiment of the invention, the delta sigma converter 10 comprises aninput transformer 19 receiving the analog signal 13 and outputting atransformed voltage to a summation point 20. The input transformer 19includes a switchable capacitor configuration which may be operated asdescribed later with reference to FIG. 4.

A feedback voltage from a feedback loop is subtracted from thetransformed voltage in the summation point 20, and the resulting signalis supplied to an integrator 21 performing a time integration of thesignal voltage from the summation point 20. The integrator 21 will havea low pass filtering effect. The integral signal provided as the outputfrom the integrator 21 will increase or decrease depending on whetherthe signal voltage from the summation point 20 is positive or negative.

The integral signal from the integrator 21 is presented to the input ofa comparator 22 for generating a logical “1”-level whenever the integralsignal exceeds a reference voltage V_(ref) presented to the comparator22, and a logical “0”-level whenever the integral signal from theintegrator 21 is below the reference voltage V_(ref). By using a batterysupply dependent reference voltage, the dynamic range and clipping levelin the A/D converter, the delta sigma converter 10, may becomeincreased. With a suitable microphone 18 the hearing aid will be able tohandle a larger dynamic range.

The reference voltage V_(ref) is adapted to be lower than the powersupply voltage provided by the battery voltage V_(battery) and to followthe decay of the battery voltage V_(battery) with a predefined margin.This predefined margin may in one embodiment be lower than 10% of thebattery voltage V_(battery). This predefined margin may in a furtherembodiment be lower than 5% of the battery voltage V_(battery).

The binary output from the comparator 22 feeds the data input of a Dflip-flop 24. The D flip-flop 24 captures the value of the D-input at adefinite portion of the clock cycle, such as the rising edge of a clocksignal. That captured value of the D-input becomes the Q output untilthe next definite portion of the clock cycle occurs and a new value ofthe D-input is captured and becomes the next Q output.

The clock frequency of the clock signal from a clock signal generator 25defines the bit rate of the output signal 14 from the delta sigmaconverter 10. In the illustrated embodiment the clock frequency isstable in the range of 1-2 MHz. A charging and discharging period of theinput transformer 19 may correspond to e.g. 64 clock cycles from theclock signal generator 25.

The bit stream from the flip-flop 24 is provided at the output 12 of thedelta sigma converter 10 as a digital audio signal to a digital signalprocessor 27. The digital signal processor 27 is preferably aspecialized microprocessor with its architecture optimized for theoperational needs of digital signal processing, and in the illustratedembodiment the processor 27 is adapted for amplifying and conditioningof the audio signal intended to become presented for the hearing aiduser. The amplification and conditioning is carried out according to apredetermined setting in order to alleviate a hearing loss by amplifyingsound at frequencies in those parts of the audible frequency range wherethe user suffers a hearing deficit.

The processor 27 outputs according to one embodiment of the invention adigital signal fed to a digital output stage 28 and an output transduceror a speaker 29. The speaker 29 may be driven as a class D amplifier bythe one-bit digital data stream received.

The output 12 of the delta sigma converter 10 is branched to provide apart of the data stream to a feedback loop. In the feedback loop, thepart of the data stream is forwarded to a 1-bit D/A converter 26converting the logical ones and zeroes in the part of the data streaminto a positive or negative voltage with respect to the transformedvoltage for subtraction from the transformed voltage in the summationpoint 20.

The gain in the A/D converter or the delta sigma converter 10 isinversely proportional to the reference voltage, V_(ref). This isopposite to the output stage 28, which has a gain proportional to thereference voltage, V_(ref). The entire hearing aid or hearing assistivedevice is neutral in relation to the reference voltage, V_(ref), and wethereby obtain an additional advantage using a reference voltage,V_(ref), following the decay of the power supply voltage V_(battery)with a predefined margin.

An embodiment of the reference voltage generation circuit 23 is shown inFIG. 3. The reference voltage generation circuit 23 is adapted toprovide a reference voltage V_(ref) from a power supply V_(battery),which may be hearing aid battery cell, e.g. of type 312 (discharge curveindicated in FIG. 5. The reference voltage generation circuit 23comprises an electronic voltage amplifier or op-amp being coupled to thepower supply V_(battery) via a passive circuit. The electronic voltageamplifier (op-amp) outputs the reference voltage V_(ref). The referencevoltage generation circuit 23 is adapted to control the referencevoltage V_(ref) to be lower than the power supply V_(battery) and tofollow the decay of the power supply V_(battery) with a predefinedmargin ΔV.

The passive circuit of the reference voltage generation circuit 23includes two resistors R₁ and R₂ providing a first voltage divider. Thefirst voltage divider in the illustrated embodiment is a simpleconfiguration with two resistors connected in series, with the inputvoltage V_(battery) applied across the resistor pair and the outputvoltage emerging or being tapped from the connection between them. Theoutput voltage from the first voltage divider is connected to a firstinput terminal of the electronic voltage amplifier via a low pass filterprovided by a diode D₁ and a capacitor C₁. The low pass filter removesnoise originating from the power supply, and prevents rapid changes ofthe voltage that would generate audible artifacts.

Two resistors R₃ and R₄ provide a simple configuration for the secondvoltage divider with the two resistors connected in series. Thereference voltage V_(ref) is applied across the resistor pair and theoutput voltage emerging from the connection between them. The outputvoltage from the second voltage divider is connected to a second inputterminal of the electronic voltage amplifier. As the electronic voltageamplifier ensures that the voltage on its two input terminals areidentical, the reference voltage V_(ref) may be expressed as follows:

V _(ref) =V _(battery)*(1+R ₃ /R ₄)/(1+R ₁ /R ₂)

The reference voltage V_(ref) must be less than the battery supplyV_(battery) so the gain must be configured so that

(1+R ₃ /R ₄)/(1+R ₁ /R ₂)<1

FIG. 4 shows how the input transformer 19 is operated in two phases; acharging phase and a discharging phase. In the charging phase, theanalog signal 13 (see FIG. 1) is charging two capacitors, C_(a) andC_(b), arranged in parallel, while the same two capacitors, C_(a) andC_(b), are switched into series in the discharging phase and beingconnected to OUT or the summation point 20. The benefit by switching thecapacitor coupling between parallel and series coupling is that thevoltage supplied to the summation point 20 when switching to dischargephase is doubled and subsequently the discharging is twice as fast. Thisis beneficial as the relative noise voltage level is reduced without theneed for increasing the supply current to the amplifier in the deltasigma converter 10.

Five switching transistor S₁-S₅ are controlled by a sampling clocksignal (not shown) where the signal edge of the clock signal goespositive in the charging phase, the switching transistors S₁, S₃, and S₅close, and S₂ and S₄ open. When the signal edge of the sampling clocksignal goes negative in the discharging phase, the switching transistorsS₁, S₃, and S₅ of the input transformer 19 open, and the switchingtransistors S₂ and S₄ close.

TABLE 1 S₁, S₃, S₅ S₂, S₄ Charging phase Closed Open C_(a) and C_(b) areconnected in parallel Discharging Open Closed C_(a) and C_(b) areconnected in series phase

The filled and unfilled squares at the gate of the switching transistorS₁-S₅ indicate the operation of the switch. A filled square denotes aclosed transistor switch in the charging phase and an open transistorswitch in the discharging phase. An unfilled square denotes an opentransistor switch in the charging phase and a closed transistor switchin the discharging phase.

The noise generated from the A/D converter or the delta sigma converterwill be substantially independent of the reference voltage. The reasonfor this is that the inherent thermal noises from amplifiers etc. arethe dominant noise sources independent of the reference voltage. Ingeneral, the A/D converter or the delta sigma converter will havebenefits of having as high reference voltage as possible in order toincrease the achievable dynamic range. Hereby the range for the inputvoltage for the converter increases, but the noise remains constant. Fora hearing aid or a hearing assistive device this means that with asuitable microphone the hearing aid will be able to handle a largerdynamic range.

The reference voltage according to the invention may be used as supplynot only for the A/D converter or the delta sigma converter. Similar tothe A/D converter or the delta sigma converter, some microphone typesapplicable for use in the hearing aid or a hearing assistive device maybenefit from an increased reference voltage for improving the dynamicrange.

In order to generate a quiet and noise free reference voltage withadequate rejection of noise from the voltage supply (battery), thecircuit that generates the reference voltage requires some voltageheadroom. FIG. 5 illustrates (voltage versus time) how the voltage 40from a hearing aid battery (type 312) varies during its lifetime. Ittypically starts out at a voltage up to 1.4 Volts, but the voltage candecrease to less than 1.0 volts before the battery is no longer able topower the hearing aid or the hearing assistive device. Therefore, thereference value has traditionally been decided to be fixed at a leveljust below the voltage at which the battery is no longer able to powerthe hearing aid or a hearing assistive device allowing at least 50 mV ofvoltage headroom for noise removal.

According to the invention, the reference voltage generation circuit isadapted to provide a reference voltage V_(ref) which is lower than apower supply voltage V_(battery) and following the decay of the powersupply voltage V_(battery) with a predefined margin. The referencevoltage V_(ref) is supplied to the A/D converters and microphones. Thismay increase the achievable dynamic range during most of the battery'slifetime.

The reference voltage generation circuit 23 according to one embodimentof the invention provides a reference voltage V_(ref) from a powersupply V_(battery). The reference voltage generation circuit is adaptedto control the reference voltage V_(ref) (the curve 42) to be lower thanthe power supply V_(battery) (the curve 40) and to follow the decay ofthe power supply V_(battery) with a predefined margin ΔV (the difference43). The level 41 is indicated on the figure as the level traditionallyused to power the A/D converter and ensuring quiet and noise freereference voltage.

1. A hearing assistive device including an audio processing circuitcomprising: an analog-to-digital converter having an integratorintegrating a voltage present in a summation point; a comparatorcomparing a voltage output from the integrator with a reference voltage(V_(ref)) and outputting a logical level in accordance with thecomparison; a feedback loop coupling a feedback signal back to thesummation point; and a reference voltage generation circuit beingadapted to provide the reference voltage (V_(ref)) at a level lower thanthat of a power supply voltage (V_(battery)) and following the decay ofthe power supply voltage (V_(battery)) with a predefined margin.
 2. Thehearing assistive device according to claim 1, wherein the referencevoltage generation circuit comprises an electronic voltage amplifier(op-amp) coupled to the power supply (V_(battery)).
 3. The hearingassistive device according to claim 2, wherein the reference voltagegeneration circuit includes a first voltage divider (R₁, R₂), whereinthe power supply (V_(battery)) is input for the first voltage divider(R₁, R₂), and wherein the output from the first voltage divider (R₁, R₂)is connected to a first input terminal of the electronic voltageamplifier (op-amp).
 4. The hearing assistive device according to claim2, wherein a second voltage divider (R₃, R₄) receives the referencevoltage (Vref) as input, and wherein the output from the second voltagedivider (R₃, R₄) is connected to a second input terminal of theelectronic voltage amplifier (op-amp).
 5. The hearing assistive deviceaccording to claim 3, wherein the electronic voltage amplifier (op-amp)outputs the reference voltage (V_(ref)).
 6. The hearing assistive deviceaccording to claim 1, wherein the reference voltage generation circuitincludes a first voltage divider (R₁, R₂) and a second voltage divider(R₃, R₄), wherein the power supply voltage (V_(battery)) is input forthe first voltage divider (R₁, R₂), and the reference voltage (V_(ref))is input for the second voltage divider (R₃, R₄), and wherein theoutputs from the first voltage divider (R₁, R₂) and the second voltagedivider (R₃, R₄) are connected to respective input terminals on theelectronic voltage amplifier (op-amp).
 7. The hearing assistive deviceaccording to claim 6, wherein the output from the first voltage divider(R₁, R₂) is connected to the input terminal on the electronic voltageamplifier (op-amp) via a low pass filter (D₁, C₁).
 8. The hearingassistive device according to claim 6, wherein the relationship betweenthe reference voltage (V_(ref)) and power supply (V_(battery)) iscalculated from the equation:V _(ref) =V _(battery)*(1+R ₃ /R ₄)/(1+R ₁ /R ₂)
 9. The hearingassistive device according to claim 1, wherein a low pass filter (D₁,C₁) is arranged prior to an electronic voltage amplifier (op-amp) forremoving noise from the power supply (V_(battery)).
 10. The hearingassistive device according to claim 9, wherein the electronic voltageamplifier (op-amp) is a DC-coupled high-gain electronic voltageamplifier with an input and a single-ended output presenting thereference voltage (V_(ref)).
 11. The hearing assistive device accordingto claim 1, wherein the analog-to-digital converter is a delta sigmaconverter comprising: an input transformer receiving an input voltageand outputting a transformed voltage to a summation point; and whereinthe input transformer includes a switchable capacitor configuration. 12.A reference voltage generation circuit for providing a reference voltage(V_(ref)) from a power supply (V_(battery)), and comprising anelectronic voltage amplifier (op-amp) coupled to the power supply(V_(battery)) via a passive circuit, and being adapted to control thereference voltage (V_(ref)) to be lower than the power supply voltage(V_(battery)) and follows the decay of the power supply voltage(V_(battery)) with a predefined margin.
 13. The reference voltagegeneration circuit according to claim 12, wherein the passive circuitincludes a first voltage divider (R₁, R₂), wherein the power supply(V_(battery)) is input for the first voltage divider (R₁, R₂), andwherein the output from the first voltage divider (R₁, R₂) is connectedto a first input terminal of the electronic voltage amplifier (op-amp).14. The reference voltage generation circuit according to claim 13,wherein a second voltage divider (R₃, R₄) receives the reference voltage(Vref) as input, and wherein the output from the second voltage divider(R₃, R₄) is connected to a second input terminal of the electronicvoltage amplifier (op-amp).
 15. The reference voltage generation circuitaccording to claim 12, wherein the passive circuit includes a firstvoltage divider (R₁, R₂) and a second voltage divider (R₃, R₄), wherethe power supply (V_(battery)) is input for the first voltage divider(R₁, R₂), and the reference voltage (V_(ref)) is input for the secondvoltage divider (R₃, R₄), and wherein the outputs from the first voltagedivider (R₁, R₂) and the second voltage divider (R₃, R₄) are connectedto respective input terminals on the electronic voltage amplifier(op-amp).
 16. The reference voltage generation circuit according toclaim 15, wherein the output from the first voltage divider (R₁, R₂) isconnected to the input terminal on the electronic voltage amplifier(op-amp) via a low pass filter (D₁, C₁).
 17. The reference voltagegeneration circuit according to claim 15, wherein the relationshipbetween the reference voltage (V_(ref)) and power supply (V_(battery))is calculated from the equation:V _(ref) =V _(battery)*(1+R ₃ /R ₄)/(1+R ₁ /R ₂)
 18. The referencevoltage generation circuit according to claim 12, wherein the electronicvoltage amplifier (op-amp) is a DC-coupled high-gain electronic voltageamplifier with a input and a single-ended output presenting thereference voltage (V_(ref)).
 19. A method for processing audio in ahearing assistive device, and comprising steps of: converting an analogsignal from a microphone in an analog-to-digital converter; generating areference voltage (V_(ref)) from a power supply voltage (V_(battery)),the reference voltage (V_(ref)) being lower than the power supplyvoltage (V_(battery)), and following the decay of the power supplyvoltage (V_(battery)) with a predefined margin; and supplying thereference voltage (V_(ref)) to the analog-to-digital converter.
 20. Themethod according to claim 19, wherein the analog-to-digital converter isa delta sigma converter having a comparator, and comprising steps of:comparing an output from an integrator with the reference voltage(V_(ref)), and outputting a logical level in accordance with thecomparison.